If you have a bonus on your account, you will see "Bonus available" with a small gift symbol on the betting slip below the amount entry.
In addition, a little further down there is a slider "Use bonus" with a small gift symbol. You can use the bonus to play by moving the slider to the right under "Use bonus".
However, the gift symbol "Bonus available" and the slider "Use bonus" are only displayed if you fulfil the bonus conditions. I.e. if the bonus is only valid for football matches, but you have a basketball match on your betting slip, then you will not see a gift symbol or cannot activate the "Use bonus" slider.
Please note that the possible winnings are reduced if you play with bonus credits. When betting with bonus credits, only the winnings that exceed the stake are credited. The bonus credit stake is not refundable. You can find more information about this under the point "Use of bonus credits".